EFFKfTS OF SNAK1-: \i:i\()MS CX DOMESTIC ANIMALS. 339 



a narrow neck, an elliiitical jjnpil, and the siiort stumpy tail. l)ut 

 these points are not constant, and a tm-ther few Cohihrin;e also 

 exhibit these features. 



The Xij^eridtX? are divided into two suij-famihes, viz., \'iper- 

 iucE and Crotalin?e. These are distinguished from each other by 

 the presence of a laroe sensory i)it in the Crotahnx, named the 

 loreal pit, which is a i)aireil fossa placed betvreen the eye and the 

 nostril. 



In the classification of snal:es it must be remembered that 

 colour is a very unreliable gtiide, as markings and coloiu' may vary 

 considerably in the same s])ecies in different localities. 



Tiri". \'i:NoM Ari'ARATUS. 



Venom is secreted b_\- a ])air of glands which are placed at the 

 position of the parotids of other animals, and with which they 

 are reo;arded as homologus. All snakes possess glands in this 

 position, but the glands of the poisonous varieties are much more 

 developed, particularly in their posterior extremity. These glands 

 are surrounded by a fibrous capsule, and are covered by the 

 masseter muscle, which has an insertion into the capsule, so that 

 in the process of biting, the gland is forcibly compressed. Con- 

 nected to each gland, and enclosed in its capsule, is an excretory 

 duct which runs along the outer side of the upper jaw, and termi- 

 nates in a slit-like o])ening at the l)ase of the ])oison fang. This 

 oj^ening is in some snakes controlled b}' a si)hincter muscle, and 

 is in contact with, l)nt not attached to, the lov/er opening in the 

 poison fang. The venom is expressed from the gland at the 

 moment of biting by the contraction of the neighboiunng muscles, 

 passes along the duct, entering the canal in the fang, and is 

 injected into the r.art l)itten. Not only is this expulsion due to 

 muscular contraction of the muscles surrounding the gland. l)ut 

 in some snakes it would appear tb.at the gland substance itself 

 contained some nuiscular fibres. Thus, if the glands of the night- 

 adder iCaiisiis rlioinbcalns) 1)e excised, and the venom allowed 

 to flow out by holding the gland l)y the apex, it will be noticed 

 that in a few moments a series of muscular contractions will set 

 it. causing the ribbon-like gland to become distorted, and the 

 venom will be freely expressed. 



The fang is a modificaticjn of the grooved tooth previously 

 referred to. the open groove on the anterior surface having now 

 become — by an infolding and fusion of its free border.s — a canal 

 with the ends only open. In some of the Hydropin?e the fusion 

 of the edges is not complete, l)Ut in the other Thanatophidia the 

 only o]:>enings present are one near the base to which the end of 

 the orifice of the poison duct is a])])lied, and another near the free 

 extremity of the fang. 



Qi:ANTi'r\ oi" Venom. 



The amount of venom which can be obtained at one time 

 from a snake depends on a number of factors, vie, the species, 

 condition and size of the indiviflual, whether the animal is fasting 



